Add a wrapper around SystemClock.uptimeMillis().

This bug is motivated by bug 140208026 where we want to replace
Thread.sleep() with TestLooper.moveTimeForward() in PackageWatchdogTest.java.

However, it turns out that PackageWatchdog uses SystemClock.uptimeMillis()
internally. The tests will fail if we don't forward PackageWatchdog's internal
clock accordingly.

We add a wrapper around SystemClock.uptimeMillis() so it is customizable
by the test case.

Bug: 140358475
Test: atest PackageWatchdogTest
Change-Id: Id26325a93dc4050c6468502347b0e7852ed1263f